# A student reacts calcium with chlorine gas. Calcium is in Group 2 of the periodic table and chlorine is in Group 7. Explain why calcium and chlorine form ions, state the charges on the ions formed, and explain how these charges determine the formula of calcium chloride.

> Eduqas A-Level Chemistry — 3.5 Aromatic chemistry (A-Level only) · Explain · 5 marks

## Mark scheme (5 marks)

1. Calcium loses two electrons to form a Ca²⁺ ion (because it is in Group 2) / atoms lose or gain electrons to form ions
2. Chlorine gains one electron to form a Cl⁻ ion (because it is in Group 7)
3. Atoms form ions to achieve a full outer shell of 8 electrons / to gain a stable electron arrangement
4. The charges on the ions must balance / the overall charge of the compound must be zero
5. Two Cl⁻ ions are needed to balance one Ca²⁺ ion, giving the formula CaCl₂

## Key terms

- [ion](https://www.gradenine.co.uk/glossary/ion)

## Related

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-reacts-calcium-with-chlorine-63afd3a2) · Published by Druglandscape Ltd.